Infants born into different language communities are exposed to languages of widely different structures. Yet, sooner or later, every normal child masters the structure to which he has been exposed. The set of abilities which the child brings to bear on this task has been called the Language Acquisition Device (LAD). The particular version of LAD given in this proposal is to be expressed as a computer simulation capable of accounting for the acquisition of morphophonology (the system of alterations of inflections and roots in complex words). Given a set of inflected words as input, the computer simulation should be able to attach correct inflections onto novel roots and provide these new words as outputs. Moreover, the errors produced by the simulation should occur in a sequence which bears a good correspondence to the sequence of errors made by the language learning child. The simulation should be applicable to the acquisition of morphophonological forms in any human language. The simulation will first be applied to experimental data on German and Hungarian morphophonological development that have already been collected. Then new experimental data will be gathered on the acquisition of morphophonology by Navaho and Tagalog children. These data will also be compared to parallel data generated by the simulation. Individual aspects of the simulation will be tested in a series of experiments with miniature linguistic systems. Finally the simulation will be extended to account for new lognitudinal data on the acquisition of syntax.